The present invention relates to a tire having a radial carcass reinforcement, and more particularly to a tire intended to be fitted on vehicles bearing heavy loads and traveling at sustained speed, such as, for example, lorries, tractors, trailers or highway buses.
Generally, the carcass reinforcement is anchored on either side in the zone of the bead and is radially surmounted by a crown reinforcement formed of at least two plies, which are superposed and formed of cords or cables which are parallel within each ply. It also generally comprises a ply of metal wires or cables of low extensibility which form an angle of between 45° and 90° with the circumferential direction, this ply, referred to as a triangulation ply, being radially located between the carcass reinforcement and the first, so-called working, crown ply, formed of parallel wires or cables having angles at most equal to 45° in absolute value. The triangulation ply forms with at least said working ply a triangulated reinforcement, which undergoes little deformation under the different stresses to which it is subjected, the essential role of the triangulation ply being to take up the transverse compressive stresses to which all the reinforcement elements in the zone of the crown of the tire are subjected.
The crown reinforcement comprises at least one working ply; when said crown reinforcement comprises at least two working plies, these are formed of inextensible metallic reinforcement elements, which are parallel to each other within each ply and crossed from one ply to the next, forming angles of between 10° and 45° with the circumferential direction. Said working plies, which form the working reinforcement, may also be covered by at least one so-called protective ply, formed of advantageously metallic, extensible reinforcement elements, which are referred to as elastic elements.
In the case of tires for “heavy vehicles”, a single protective ply is usually present and its protective elements are in most cases oriented in the same direction and at the same angle in absolute value as those of the reinforcement elements of the radially outermost and hence radially adjacent working ply. In the case of construction-vehicle tires which are intended to travel on relatively bumpy roads, it is advantageous for two protective plies to be present, the reinforcement elements being crossed from one ply to the next and the reinforcement elements of the radially inner protective ply being crossed with the inextensible reinforcement elements of the radially outer working ply adjacent to said radially inner protective ply.
Cables are said to be inextensible when said cables have a relative elongation at most equal to 0.2% under a tensile force equal to 10% of the breaking load.
Cables are said to be elastic when said cables have a relative elongation at least equal to 4% under a tensile force equal to the breaking load.
The circumferential direction of the tire, or longitudinal direction, is the direction corresponding to the periphery of the tire and defined by the direction of rolling of the tire.
The transverse or axial direction of the tire is parallel to the axis of rotation of the tire.
The radial direction is a direction intersecting and perpendicular to the axis of rotation of the tire.
The axis of rotation of the tire is the axis around which it rotates in normal use.
A radial or meridian plane is a plane containing the axis of rotation of the tire.
The circumferential median plane, or equatorial plane, is a plane which is perpendicular to the axis of rotation of the tire and divides the tire into two halves.
Certain current tires, referred to as “highway” tires, are intended to travel at high speed and on increasingly long journeys, owing to the improvement in road networks and the growth in motorway networks throughout the world. All the conditions under which such a tire is required to travel without doubt make it possible to increase the number of kilometers traveled, the wear of the tire being less; on the other hand, the endurance of the latter, and in particular of the crown reinforcement, is greatly impaired.
The stresses existing at the level of the crown reinforcement, and more particularly the shearing stresses between the crown plies, combined with a non-negligible increase in the operating temperature at the ends of the axially shortest working ply, result in the appearance and propagation of cracks in the rubber at said ends, despite the presence of a thickened layer of rubber at the junction of the edges of the working crown plies. The same problem exists in the case of edges of two plies having reinforcement elements, said other ply not necessarily being radially adjacent to the first.
The progress made in terms of length of wear life (number of kilometers traveled) of “heavy-vehicle” tires, and also the possibility of easy, economical subsequent retreading, requires a crown reinforcement, the resistance to separation between the edges of working plies of which is improved.
In order to overcome the above disadvantages and to improve the life of the crown reinforcement of the type of tire in question, a certain number of prior patents claim solutions relating to the structure and quality of the layers and/or profiled elements of rubber mixes which are arranged between and/or around the ends of plies, and more particularly the ends of the axially shortest working ply.
French Patent 1 389 428, to improve the resistance to degradation of the rubber mixes located in the vicinity of the edges of the crown reinforcement, advocates the use, in combination with a tread of low hysteresis, of a rubber profiled element covering at least the sides and the marginal edges of the crown reinforcement and formed of a rubber mix of low hysteresis.
French Patent 2 222 232, in order to avoid separations between crown reinforcement plies, teaches coating the ends of the reinforcement with a pad of rubber, the Shore A hardness of which differs from that of the tread surmounting said reinforcement, and is greater than the Shore A hardness of the profiled element of rubber mix arranged between the edges of crown reinforcement plies and the carcass reinforcement.
U.S. Pat. No. 5,154,217 uses a different unit of measurement, and claims arranging between the ends of two plies, in the extension of the ply between said two plies, a pad of rubber mix, the elasticity modulus of which at 100% elongation is greater than the modulus of the same name of the tread.
To effect joining of the respective two edges of two crown reinforcement plies, French Patent 2 298 448 discloses the use of so-called shearing rubbers between said edges, of a high Shore A hardness and elasticity modulus at 100%, this use being combined with the use of anisotropic rubber strips arranged between the edges of the crown reinforcement and the carcass reinforcement.
The same applies to the case of joining the crown reinforcement plies described in French Patent 2 499 912, the lateral part of the rubber layer arranged between the two main plies of the crown reinforcement being formed of a rubber mix of high Shore hardness.
Other solutions have been considered for improving the resistance to separation of the crown reinforcement plies, which solutions consist of coating at least the end of the axially least wide ply in at least one layer of rubber mix, whether or not reinforced by reinforcement elements. In the case of a single layer, the latter is advantageously turned up over the end of the ply, as described and illustrated, for example, in document FR 1 226 595, in which the layer is reinforced by very fine metal wires, or in document JP 266 703, the protective layer being formed solely of rubber of a high elasticity modulus, or alternatively in French Patent 2 671 516.
It was noted that the various structures listed above did not provide a completely satisfactory solution under conditions of travel which are highly disadvantageous to the tire.
In Patent EP 1 062 106, the Applicant proposed a tire, the crown reinforcement of which comprises at least two plies of reinforcement elements, in which the edge of the axially least wide ply is separated from the axially widest ply by a profiled element of rubber mix, the axially outer end of which is located at a distance from the equatorial plane of the tire which is at least equal to the distance between said plane and the end of the widest ply and said profiled element itself being separated from the liner of the least wide ply by an edging rubber, said profiled element, said edging rubber and said liner having respectively secant modules of elasticity under tension at 10% relative elongation such that they decrease radially towards the inside from the liner to the profiled element.
The tests performed with this type of tire show that in view of current demands relating to the applications in question, it is still necessary to improve the performance of the tires, in particular in terms of endurance.
The inventors have thus set themselves the task of producing tires intended to be fitted on vehicles bearing heavy loads and traveling at sustained speed, having improved endurance compared with known tires, and in particular of limiting the appearance and propagation of cracks at the end of the axially narrowest working ply.